The present invention relates to a radio communication system in which a voice call is capable of being made among a plurality of radio personal stations (portable terminals) and in which a plurality of radio personal stations (portable terminals) are capable of being informed of information such as a voice and various data from a central server.
In recent years, radio communication systems have developed remarkably. A conventional radio communication system will be described below.
For example, there is conceivable a local voice communication system in which in a large scale shopping center, store clerks in the store have radio personal stations, respectively, and the in-store business is carried out while the store clerks communicate with one another through voice calls or a supervisor who should direct and supervise all of the store clerks communicates with the store clerks. In this connection, a plurality of radio cell stations (base stations) are installed in a plurality of locations in the shopping center and are connected through wires to main equipment which has an exchange function and a control function.
The radio cell stations installed in the plurality of locations in the shopping center transfer the information such as voice and data to the radio personal stations by utilizing the TDMA (Time Division Multiple Access) method. Both of a slot and a carrier frequency which are to be used are managed by a radio channel managing unit provided in the radio cell station. In addition, the main equipment is connected to a multi-media server from which the voice and the data are sent onto a radio line through the main equipment and the radio cell station.
Now, a description will be given with respect to an information monitoring state in which the information such as voice and data sent from the multi-media server is monitored at all times by the radio personal stations. In accordance with the instruction issued from the main equipment, the radio cell stations simultaneously inform onto the radio lines the voice and the data which are presented by the multi-media server. In this connection, both of the carrier frequency and the slot which are used in the simultaneous information are stored in the radio channel managing unit provided in each of the radio cell stations.
In order to carry out the simultaneous information, each of the radio cell stations determines the usable carrier frequency and slot in accordance with the electric wave situation thereof. Then, each of the radio cell stations transmits the information by radio in such a way that the determined carrier frequency and the determined slot do not overlap with those which other radio cell stations use. The information exhibiting the carrier frequency and the slot which are determined and are used at this time is stored in the radio channel managing unit of each of the radio cell stations. At the time when starting the simultaneous information, each of the radio cell stations transmits the information of the carrier frequency and the slot, both of which are managed in the radio channel managing unit, to the radio personal stations through the control channel (CCH).
Each of the radio personal stations receives the control channel (CCH) transmitted by the radio cell station in the system thereof, and receives the channel information (the slot and frequency information) of the simultaneous information contained on the control channel (CCH). When the channel information of the simultaneous information can be received, the reception operation is started with the slot and the carrier frequency, so that the radio personal station proceeds to the monitoring state of the simultaneously informed information.
When the radio personal station is in the monitoring state and a user of the radio personal station carries out the manipulation for a call, the radio personal station carries out the line connection procedure by transmitting through the control channel (CCH) the assignment request which gets the associated radio cell station to assign thereto a traffic channel (TCH). On the other hand, in the associated radio cell station as well, the line connection procedure is carried out to set the channel between the radio personal station of interest and the associated radio cell station. As a result, the radio personal station of interest proceeds to the one-to-one correspondence call state.
Thereafter, when the user carries out the manipulation for quitting the call, the disconnection restoration sequence is executed, so that the radio personal station of interest makes the transition to the information monitoring state again. At this time, the radio personal station of interest receives the channel information (the slot and frequency information) for the simultaneous information contained on the control channel (CCH) of interest by searching the control channel (CCH) which is intermittently transmitted by the radio cell station of the system thereof at all times.
After having received the channel information of the simultaneous information, the radio personal station of interest starts only the reception operation with the slot and the carrier frequency to proceed to the information monitoring state (the state in which the simultaneous broadcasting, the local broadcasting and the like can be received) again.
In the above-mentioned conventional radio communication system, however, in the case where the call request and the like occur after starting the information monitoring, in order that the radio personal station of interest may proceed to the monitoring state again, the radio personal station of interest needs to start with the open search of the control channel to obtain the channel information (the slot and carrier frequency information) with which the simultaneous information is carried out. Hence, a time period of several seconds may be demanded in some cases. As a result, in such cases, the radio personal station of interest can not be immediately switched over to the monitoring state. This is a problem.
Now, it is conceivable that the radio communication system is designed in such a way that even when the transition is made from the information monitoring state to the call state, the simultaneously informed information is transmitted through the line which is being used in the call. In this case, even when the user is in call, he/she can grasp the contents of the simultaneously informed information. This is a right matter. On the other hand, in such a configuration, if it takes a time period of several seconds to obtain the channel information when the radio personal station of interest proceeds to the monitoring state again, then the simultaneously informed information will be necessarily interrupted for this time period of several seconds. As a result, there arises a problem that the user misses listening to the contents of the simultaneously informed information during that time period.